1. Field of the Invention
This invention relates to a semiconductor device and a manufacturing method thereof, and more particularly to formation of a composite insulative film by use of the CVD (Chemical Vapor Deposition) method.
2. Description of the Related Art
In a semiconductor integrated circuit, for example, an insulative film used for forming a capacitor of a memory cell is required to be made thin and have a large dielectric constant in order to stably provide a preset capacitance even when the size of the capacitor is reduced with an increase in the integration density. A silicon nitride film is known as an insulative film having a dielectric constant larger than that of a silicon oxide film.
FIG. 18 is a cross sectional view showing a semiconductor device manufactured by the conventional manufacturing method. An insulative film 2 is formed on the surface of a semiconductor wafer 1 and patterned. A trench 3 is formed in the semiconductor wafer 1 by etching the semiconductor wafer 1 with the insulative film 2 used as a mask. Next, a single crystal Si layer doped with impurity, that is, an impurity layer 4 is formed on the inner surface of the trench 3. After this, the semiconductor wafer 1 is subjected to the cleaning process by use of chemicals.
Next, the semiconductor wafer 1 is loaded into a furnace (not shown). In the furnace, a CVD silicon nitride film 6 is deposited on the inner surface of the trench 3 and the insulative film 2 by the LPCVD (Low Pressure Chemical Vapor Deposition) method. Next, a silicon oxide film 7 is formed on the silicon nitride film 6. As a result, a composite insulative film 9 formed of the silicon nitride film 6 and silicon oxide film 7 is formed in the trench 3. After this, a polysilicon film 8a doped with impurity is deposited on the silicon oxide film 7. The internal portion of the trench 3 is filled with the polysilicon film 8a. Next, the polysilicon film 8a, silicon oxide film 7 and silicon nitride film 6 are patterned to form an electrode 8 of the polysilicon film 8a in the trench 3. Therefore, a capacitor 10 is constructed by the electrode 10, the composite insulative film 9 and the insulative diffusion layer 4 used as an electrode in the trench 3.
When the silicon nitride film 6 is formed on the inner surface of the trench 3 in the furnace, a natural oxide film 5 with a thickness of approx. 1 nm to 2 nm is formed between the silicon nitride film 6 and the impurity diffusion layer 4. Therefore, the actual composite insulative film 9 is formed of the natural oxide film 5, silicon oxide film 7 and silicon nitride film 6. As the cause of formation of the natural oxide film, the following three causes are considered. The first cause is based on dissolved oxygen in the cleaning liquid in the cleaning process, the second cause is based on oxygen in the atmosphere, and the third cause is based on oxygen contained in the outside air introduced into the silicon nitride film forming furnace when the wafer 1 is carried into the furnace.
Thus, if the natural oxide film 5 with a thickness of approx. 1 nm to 2 nm is formed between the silicon nitride film 6 and the impurity diffusion layer 4, the film thickness of the capacitor insulative film is increased by 1 nm to 2 nm, thereby preventing the film thickness of the capacitor insulative film from being reduced. Further, if the natural oxide film is formed, the film quality of the capacitor insulative film is deteriorated, thereby lowering the dielectric strength and degrading the reliability of the insulative film.
As a method of solving the above problems, a method of removing the natural oxide film 5 in the trench 3 by forming an atmosphere in which the partial pressures of H.sub.2 O and O.sub.2 are set equal to or lower than 1.times.10.sup.-4 Torr in the furnace used for the LPCVD method, processing the semiconductor wafer 1 in the atmosphere, and then forming the CVD silicon nitride film 6 is considered.
Further, a method of reducing the natural oxide film 5 by introducing a reducing gas such as H.sub.2, SiH.sub.4, Si.sub.2 H.sub.6, and HCl into the furnace used for the LPCVD and then forming a thermal nitride film is considered.
However, according to the above two methods, after the natural oxide film 5 on the inner wall of the trench is removed in the furnace, the clean inner surface of the trench 3 is exposed to the high-temperature and low-pressure atmosphere so that impurity in the impurity diffusion layer 4 will be diffused towards the outside and extracted. As a result, if a composite insulative film formed by the above method is used for the capacitor insulative film of the memory cell and when an electric field is applied to the capacitor insulative film, the interface area between the capacitor insulative film and the impurity diffusion layer is depleted, thereby causing a problem that an effective storage amount of charges cannot be obtained.
Further, according to the above two methods, since the clean inner surface of the trench 3 is exposed to the high-temperature and low-pressure atmosphere after the natural oxide film 5 in the trench is removed in the furnace, etching pits may be formed in the inner surface of the trench by an oxidation gas such as H.sub.2 O or O.sub.2 in some cases. In addition, SiC may be sometimes formed on the inner surface of the trench by a carbonaceous gas such as CO, CO.sub.2 and hydro-carbon gas. Therefore, if a composite insulative film formed by the above methods is used for the capacitor insulative film of the memory cell, a problem that the dielectric strength and the reliability of the capacitor insulative film are lowered occurs.